The invention relates to improvements in vacuum double drum filters and particularly to an improved structure for removing the filtrate through the inner surface of the porous drum and improved stripping devices for the filter cake.
Double drum filters conventionally employ parallel porous horizontal drums rotating in opposite directions and positioned with their peripheral surfaces closely adjacent so that material to be filtered can be fed in the gap between them. A vacuum is applied within the filter drum to remove the filtrate and the cake forms on the outer surface and is removed from the outer surface. An example of this general type of filter is shown in U.S. Pat. No. 4,102,791, Otte, issued July 25, 1978.
Vacuum double drum filters of this general type are employed for dewatering mass flows such as in coal processing systems. In these commercial operations, the amount of filtrate which must be extracted is substantial, and it is difficult to obtain a dry filter cake.
It is accordingly an object of the present invention to provide an improved double drum filter with a structure capable of obtaining a more dry filter cake and avoiding disadvantages of structures heretofore available.
In accordance with the preferred form of the invention, the layering of coarse grain and fine grain in the filter cake layer is to be improved, and the suction zone is enlarged. Also, the amount of water which reenters or rewets the filter cake is reduced so that the water flowing through the porous surface of the drum is handled in an improved manner for complete removal from the zone of the filter cake.
A feature of a preferred form of the invention is that the drum diameter of the filter drum is greater than half of the drum length, and preferably greater than 3/4 of the drum length. Generally, in double drum filters, the drum length is greater by a multiple of the drum diameter. It has been discovered that the dryness of the filter cake in double drum filters can be significantly and unexpectedly improved when the drum length is not significantly greater than the drum diameter and the ratio of drum length to drum diameter lies in proximity of or below 2:1.
By the foregoing preferred construction, the height of the sedimentation zone of the input slurry is significantly increased so that a better layering of the different grain sizes is produced on the drum surface and a significant increase of the drum circumference relative to the length obtains a shorter extraction pipe length. The dryness of the filter cake is increased due to both of these effects.
In accordance with the preferred form of the invention, extraction pipes are provided for the filtrate with filtrate collecting pockets which are positioned adjacent the inner surface of the rotating filter drum. The collecting pockets are disposed directly under the surface of the filter drum and increase the effective aperture of the filtrate extraction pipes and particularly at the beginning of extraction, a greater amount of water can stream into the extraction pipes.
Further, in accordance with the preferred embodiment, the filtrate collecting pockets are constructed to be wedge shaped. The wedge shape is particularly favorable since it offers advantages in fabrication technology for manufacture. Also, it offers a large suction area at the drum surface, and can be easily adapted at the inside of the transition to the filtrate extraction pipes so that the wedge shaped pockets conform to the diameter of the filtrate extraction pipes.
A further feature of the invention is that the filtrate extraction pipes and/or the filtrate collection pockets are positioned at an angle with respect to the drum surface which is not at right angles, but is at an angle to the 90.degree. line drawn from the center of the drum to the drum surface. The filtration pockets are angled in the direction of rotation of the drum. Further, except for the initial pockets which carry the first filtered filtrate, the pockets are inclined relative to a horizontal position so that the residual filtrate cannot flow back to the filter surface, but will flow toward the center of the drum to be removed by the filtrate extraction pipes.
Another feature is that the filter cake stripping devices are disposed on the downrunning side of the drum below the center of the filter drums. This provides a greater effect of drum circumference for the extraction of the filtrate. The dryness of the filter cake is increased by this construction. It is also provided that the filtrate extraction pipes and the lower surface of the wedge shaped filtrate suction pockets converge into one another without bend so that a smooth discharge path is provided for the filtrate.
Other objects, advantages and features will become more apparent from the teaching of the principles of the invention in connection with the disclosure of the preferred embodiments in the specification, claims, and drawings.